1. Field of the Invention
This invention is directed to an extreme ultraviolet light source device that generates extreme ultraviolet radiation by means of plasma produced by means of discharge, and a method of generating extreme ultraviolet radiation. In particular, it concerns an extreme ultraviolet light source device that generates extreme ultraviolet radiation by means of plasma produced by means of discharge, using an energy beam to gasify high-temperature plasma raw material for the generation of extreme ultraviolet radiation when the raw material is supplied to the vicinity of the discharge electrodes, and a method of generating extreme ultraviolet radiation.
2. Description of Related Art
With the miniaturization and higher integration of semiconductor integrated circuits, there are demands for improved resolution in projection lithography equipment used in manufacturing integrated circuits. Lithography light source wavelengths have gotten shorter, and an extreme ultraviolet light source device (hereafter, EUV light source device) that emits extreme ultraviolet (hereafter, EUV) radiation with wavelengths from 13 to 14 nm, and particularly, the wavelength of 13.5 nm, have been developed as a next-generation semiconductor lithography light source to follow excimer laser equipment to meet these demands.
A number of methods of generating EUV radiation are known in EUV light source device; one of these is a method in which high-temperature plasma is generated by heating and excitation of an EUV radiation fuel and extracting the EUV radiation emitted by the plasma.
EUV light source device using this method can be roughly divided, by the type of high-temperature plasma production, into LPP (laser-produced plasma) type EUV light source devices and DPP (discharge-produced plasma) type EUV light source devices (see, “Recent Status and Future of EUV (Extreme Ultraviolet) Light Source Research,” J. Plasma Fusion Res., Vol. 79 No. 3, P219-260, March 2003, for example).
LPP-type EUV light source devices use EUV radiation from a high-temperature plasma produced by irradiating a solid, liquid, or gaseous target with a pulsed laser.
DPP-type EUV light source devices, on the other hand, use EUV radiation from a high-temperature plasma produced by electrical current drive.
A radiation fuel that emits 13.5 nm EUV radiation—that is, for example decavalent Xe (xenon) ions as a high-temperature plasma raw material for generation of EUV—is known in both these types of EUV light source devices, but Li (lithium) and Sn (tin) ions have been noted as a high-temperature plasma raw material that yields a greater radiation intensity. For example, Sn has a conversion efficiency, which is the ratio of 13.5 nm wavelength EUV radiation intensity to the input energy for generating high-temperature plasma that is several times greater than that of Xenon.
In the DPP type in recent years, a method has been proposed, in International Patent Application Publication WO 2005-025280 A2 and corresponding U.S. Patent Application Publication 2007/090304, of using a laser beam or other energy beam to irradiate and gasify solid or liquid Sn or Li delivered to the surface of electrodes to produce discharge, and then producing high-temperature plasma by means of discharge. The EUV light source device described in these publications is explained below with reference to FIG. 10 which is a cross-section of the EUV light source device shown in FIG. 1 of those publications.
Disk-shaped electrodes 114, 116 are located in a discharge space 112 where the pressure is regulated to the specified value. Electrodes 114, 116 are separated by a specified gap in a previously defined region 118, and rotate about an axis of rotation 146.
A raw material 124 produces high-temperature plasma for emitting 13.5 nm wavelength EUV radiation. The high-temperature plasma raw material 124 is a heated metal melt, and is held in a container 126. The temperature of the metal melt 124 is regulated by a temperature regulation means located in the container 126.
The electrodes 114, 116 are located such that a portion of each electrode is submerged in the container 126 that holds the metal melt. The liquid metal melt 124 that is carried on the surface of the electrodes 114, 116 is transported to the surface of the region 118 by the rotation of the electrodes 114, 116. The metal melt 124 that is transported to the surface of the region 118 (that is, the metal melt 124 that is present on the surfaces of the electrodes 114, 116 that are separated by a specified gap in the region 118) is irradiated by a laser beam 120 from a laser (not shown). The metal melt 124 that is irradiated by the laser beam 120 is gasified.
With the metal melt 124 gasified by irradiation by the laser beam 120, application of pulsed power on the electrodes 114, 116 starts a pulsed discharge in the region 118, and a plasma 122 is formed. The plasma 122, heated and excited by a large electrical current during discharge, attains a high temperature, and EUV radiation is generated from this high-temperature plasma. The EUV radiation passes through a debris trap 138 and is extracted from above in the Figure.
A pulsed power generator 148 is electrically connected to the metal melt 124 held in the container 126. The metal melt 124 is conductive, and so electrical energy is supplied from the pulsed power generator 148, through the metal melt 124, to the electrodes 114, 116 that are partially submerged in the metal melt 124.
By means of this type, Sn or Li that are solid at normal temperature are easily gasified in the vicinity of the discharge region where the discharge is generated (the space where a discharge between the electrodes is generated). That is, it is possible to supply easily gasified Sn or Li to the discharge region, and so it is possible to effectively extract EUV radiation of a 13.5 nm wavelength following discharge.
Further, in the EUV light source device described in International Patent Application Publication WO 2005-025280 A2 and corresponding U.S. Patent Application Publication 2007/090304, the electrodes are rotated, which has the following advantages:
(i) it is possible to constantly deliver new solid or liquid high-temperature plasma raw material, which is the EUV generation fuel high-temperature plasma raw material, to the discharge region; and
(ii) because the position on the surface of the electrodes that is irradiated by the laser beam and where the high-temperature plasma is generated is constantly changing, and so thermal load and erosion of the electrodes can be prevented.
Nevertheless, there are the following problems associated with the equipment indicated in the described in International Patent Application Publication WO 2005-025280 A2 and corresponding U.S. Patent Application Publication 2007/090304. That is, by means of the EUV light source device described, the surface of the electrodes is irradiated every time EUV radiation is generated. When the EUV light source device is used as a light source for lithography, EUV radiation is repeatedly generated from several kHz to several tens of kHz. Further, it often happens that the EUV light source device continues in operation all day long. Therefore, the electrodes are liable to be worn down by laser abrasion.